The present invention relates to a means for suppressing wave making resistance occurring during navigation of a ship as well as to a ship provided with the means. The invention gives quite satisfactory results when it is applied not only to high-speed small- and medium-sized ships but also to large-sized ships such as oil tankers under a requirement for a higher and higher speed.
The resistance occurring when a ship floats and navigates on water consists mainly of frictional resistance with water, wave making resistance and air resistance. While the frictional resistance amounts to almost 90% of the total resistance in a large vessel navigating at a low speed, the contribution of the wave making resistance increases in small or medium vessels navigating at a high speed, sometimes, amounting to 60% of the total resistance. On the other hand, the fraction of resistance due to form resistance and air resistance is only several percent in most ships. Therefore, it is usually undertaken that estimation of the total resistance is made by measuring the total resistance of a model ship, from which the frictional resistance obtained by calculation is subtracted, and the remaining wave making resistance is multiplied by the ratio of displacement of the actual ship to that of the model ship to estimate the wave making resistance of the actual ship to which the frictional resistance is added to obtain the desired total resistance.
The wave making resistance here implied refers to the resistance due to waves, i.e. gravitational waves, caused by the progress of a ship. The ratio of inertia to gravity or the so-called Froude number is an important factor of the wave making resistance.
As a means for decreasing the wave making resistance, it is proposed to decrease the gravity of the ship per se, and for this purpose, hitherto, it was taught in Japanese Patent Publications No. 55-15349, No. 48-35557 and elsewhere to provide the ship hull with a number of air-ejection holes on the bottom or sides near the bottom, from which compressed air is ejected into the sea to cause the hull to float toward the sea surface.
This method, however, is hardly applied to actual ships because, except for the vessels navigating while the entire hull is under water, i.e. submarines, having no free surface, generation of waves due to progress of the ship cannot be reduced when the hull is above the sea surface so that the method is ineffective for decreasing the wave making resistance. Moreover, there may be caused a risk of capsizing when the speed of the ship is increased to exceed a certain limit.
Also proposed are a method of bubbling pressurized air into the water forward in the running direction of the ship so as to reduce the water resistance by means of the mass of air bubbles (see, for example, Japanese Patent Kokai No. 60-33185) and a method of decreasing the water resistance by installing thin slender boards in parallel to the outer surface of the hull and approximately at right angle to the running direction, keeping a small clearance with the outer surface of the hull surface in water, and ejecting air from a large number of small openings on the front edge of the board.
These prior art methods, however, require auxiliary facilities such as compressors for pressurizing air to be ejected into water and unavoidable consumption of a very large quantity of energy for ejecting air. Thus, none of the prior art methods is quite satisfactory in application to actual ships.